JIDF, or JIDF detected! comes from actual JIDF (Jewish internet defence force) which is an organisation that uses social media to mobilize support for campaigns against websites and Facebook groups that promote or praise what it regards as Islamic terrorism or antisemitism. The group’s website describes the JIDF as a private, independent, non-violent protest organization representing a collective of activists.

The meme itself, comes originally from 4chans /new/ board, however it was small time there, and has only become a proper meme since the recreation of /new/ on 4chon.net.

The memes applicaton is generally used as an insult, when discussing topics relating to Israel or Jewish topics, if someone presents a pro-israeli or pro-jewish view which they disagree with, they will likely say “JIDF detected!”, implying that the poster is part of the Jewish internet defence force organisation and trying to secretly promote pro-jew views.
Of course, they probably are not JIDF, but it has become so popular on 4chons /new/ board that it is thrown around in nearly every thread, and has generally come to be used to cry at someone whenever they say anything you disagree with, by accusing them of being pro-jewish.

The meme is very large on 4chon.net, which has about 14,000 users a week, however outside of that imageboard the meme is relatively unknown.

The meme has also had spinoffs, such as MIDF (muslim internet defence force) WIDF (White internet defence force) and more, which are not even real organisations unlike JIDF, infact, it has reached the point where you can accuse anyone of being the internet defence force for whatever they seem to be promoting or defending.